The present invention relates to a method for producing foundry cores and moulds as well as moulds and cores thereby obtained.
Various methods for making foundry moulds and cores are already known. These methods are divided into two categories: so-called "hot box" methods and so-called "cold box" methods.
The hot box method relates particularly to a process in which a mixture of sand and a liquid thermo-hardening resin is used. Under the influence of heat (treatment at a temperature between 100.degree.C and 300.degree.C for a certain period), the hardening of a core or a mould is effected.
Another hot box method is the so-called "Croning" or "Shell Molding" method.
According to this method the sand is first enrobed with a novolac resin and a reticulation agent such as hexamethylenetetramine. Thermal treatment at 200.degree.C to 300.degree.C during a certain time hardens the mixture.
The main disadvantages of the hot box methods are, on one hand, the high calorie expenditure required to raise the temperature of the box to the levels indicated above for a certain time and, on the other hand, the difficulties which result from the manipulation of the boxes.
Among the cold box methods, the method that has been known for a long time is the so-called "silicate-CO.sub.2 " method.
According to this method, the foundry sand is mixed with a silicate and the core is hardened by passing carbon dioxide through the mixture of sand and silicate which causes the hardening of the binder and fixes the form of the product that is being made.
Another cold box method is the so-called ASHLAND method. The moulding method consists essentially of foundry sand to which has been added a resin which can be polymerized by the action of a catalyst, such as triethylamine or dimethylethylamine. When the mixture is introduced into the moulding cavity, a gas such as carbon dioxide to which a catalyst has been added is injected into it causing the rapid hardening of the polymerizable resin incorporated in the mixture.
This method has many advantages (the temperature need not be raised, fast production rates can be achieved) but it also has some serious disadvantages in that the catalysts used have a disagreeable odor and even a certain toxicity which makes it necessary to take measures to prevent the hardening gas from spreading into the atmosphere, and also in that it is necessary to provide moulding tools to insure a good distribution of the hardening gas, which complicates the construction of these tools. Still in the category of cold box methods, there are also known so-called "self-hardening" methods. These methods consist in mixing a binder with the foundry sand, the binder generally being a synthetic resin, and a reagent which causes the hardening of the binder by simple contact.
These self-hardening methods are called "slow setting" methods, when the nature of the binder used and the nature or the concentration of the reagent cause hardening to take place slowly, or else they are called "rapid setting" methods, when the binder as well as the reagent cause hardening to take place rapidly.
The slow setting methods can only be used for very large components for which fast production rates cannot be envisaged.
The rapid setting methods, on the other hand, can be used for small components which have to be produced at a rapid rate, but unfortunately the duration of the life of the prepared mixture is extremely short (a few seconds) so that there is a risk of premature hardening taking place in the core and thus requiring complete emptying of the core box after each filling operation. These constraints considerably limit the applications of rapid setting or self-hardening methods.